We have used 57 Fe nuclear resonance vibrational spectroscopy (NRVS) to study the iron site in the iron-sulfur-cluster-free hydrogenase Hmd from the methanogenic archaeon Methanothermobacter marburgensis. The spectra have been interpreted by comparison with a cis-(CO) 2 -ligated Fe model compound, Fe(S 2 C 2 H 4 )(CO) 2 (PMe 3 ) 2 , as well as by normal mode simulations of plausible active site structures. For this model complex, normal mode analyses both from an optimized Urey-Bradley force field and from complementary density functional theory (DFT) calculations produced consistent results.Previous IR spectroscopic studies found strong CO stretching modes at 1944 and 2011 cm −1 , interpreted as evidence for cis-Fe(CO) 2 ligation. The NRVS data provide further insight into the dynamics of the Fe site, revealing Fe-CO stretch and Fe-CO bend modes at 494, 562, 590, and 648 cm −1 , consistent with the proposed cis-Fe(CO) 2 ligation. The NRVS also reveals a band assigned to Fe-S stretching motion at ~311 cm −1 , and another reproducible feature at ~380 cm −1 . The 57 Fe partial vibrational densities of states (PVDOS) for Hmd can be reasonably well simulated by a normal mode analysis based on a Urey-Bradley force field for a 5-coordinate cis-(CO) 2 -ligated Fe site with additional cysteine, water, and pyridone cofactor ligands. A final interpretation of the Hmd NRVS data, including DFT analysis, awaits a 3-dimensional structure for the active site.